Production of transgenic plants requires transformation of plant cells with foreign DNA and regeneration of transgenic plants from the transformed cells. Numerous methods for transformation of plant cells are known including Agrobacterium-mediated transformation, particle gun bombardment, silicon carbide whiskers, sonication or electroporation. However, successful regeneration of plants capable of transmitting foreign DNA to progeny from the transformed cells has proven difficult for numerous economically important crops.
Microparticle-mediated transformation of cotton embryos gave rise to a very low frequency of germline transformants. See PCT/US92/01721. Bombardment of meristems produced one clonal transgenic plant per 100 to 500 meristems targeted for transformation. Only 0.1 to 1% of these transformants were capable of transmitting foreign DNA to progeny. Likewise, U.S. Pat. Nos. 5,004,863 and 5,159,135 describe a method for Agrobacterium-mediated transformation of cotton that is genotype-dependent. These patent disclosures describe successful transformation of a limited number of genotypes within Gossypium hirsutum.
Somatic embryogenesis has been found to be genotype dependent in many different crop species. With regard to cotton, only a few varieties in each species of the commercially important G. hirsutum and G. barbadense can generally be transformed and regenerated by somatic embryogenesis. See Trolinder et al., Plant Cell Reports 8: 133-136 (1989) and PCT/US92/01721. Another difficulty with this approach is that growth regulators or plant hormones are required to redirect tissue differentiation. These treatments lengthen the time period required to achieve embryogenesis and may induce somaclonal variation. Trolinder et al. , 1991 Plant Cell Reports 10: 448-52 (1991). Somaclonal variation can be eliminated or minimized if regeneration occurs in a short time period.
Likewise, in order to recover a fertile plant that is resistant to a selected biotic or abiotic agent as a result of in vitro selection, a reproducible method for regeneration of plants is required. For example, recovery of a fertile plant that is resistant to a herbicide or pathogen through in vitro selection is dependent upon a reliable method for regeneration of a plant.
A need therefore exists for a method for regeneration of plants in vitro that can be utilized with diverse genotypes. A need also exists for a method for germline transformation of plants that can be utilized with diverse genotypes. In particular, a need exists for an efficient method for transformation of cotton plants of diverse genetic backgrounds. A need also exists for an efficient method for production of transformed cotton plants capable of transmitting a foreign gene to progeny.